The angular movements between objects that have one or more degrees of freedom are measurable with complex devices of various structure of mechanical, optical, electromagnetic nature, etc.
One of the fields in which the angular measures are very important is that of the study of the posture of the human body by measure and control of absolute position data of single points of the body or of angular relative position of two adjacent limbs. On the basis of the data recorded a digital model of the human body is then created.
Many are the possible applications, such as the production of digital movies or virtual reality, as well as applications in the medical field, and deserve a space of primary importance in scientific research.
However some limits exist that define substantially the fields of applicability for each type of device. Such limits are given mainly to the size of the limb that has to be monitored and to the number of degrees of freedom of the limb same.
In particular, the limbs with greater volume, such as arms and legs, have less degrees of freedom and the devices used for detecting their motion have larger weight and encumbrance and require a higher rate of precision. The limbs with smaller volume, such as the fingers of the hands, have a higher number of degrees of freedom and the devices used for detecting their motion must have lower weight and encumbrance and require a lower rate of precision.
The least expensive devices for detecting the movement of the limbs with greater volume are of mechanical type. They provide the use of rigid parts connected with rotational and prismatic joints, measuring angular movements with potentiometers. Even if the costs of this technology are low, however the rigid structure has high encumbrance, it is heavy and the measures obtained are usually not so precise.
Also magnetic sensors exist, which-require one or more transmitters for creating a magnetic field in a determined workspace. However, they have high costs and have the further drawback of being particularly affected by the presence of metal that can distort the magnetic field.
The optical sensors, finally, require optical tracers, active or reflective, whose light is captured by cameras for then analysing the position data by means of a computer. Usually, the optical devices are less bulky of the magnetic, but their correct operation can be affected by parts of the body that cross accidentally the light path. Other drawbacks of the optical devices are high costs and the need for post-processing the measured data as well as the long set-up time for the measuring equipment.
Another system of “motion capture” for the human body is described in U.S. Pat. No. 6,050,962. It provides angular sensors of resistive type or “resistive bend sensors” arranged at the joints, associated to auxiliary articulated connections formed by a plurality of links hinged in turn. The links form a chain that can rotate in a single plane. Consecutive portions of chain can rotate in different planes connected to each other by stiff or articulated junction elements. The angular sensors are in particular resistive segments that measure the rotations of the limbs to which they are applied, or of portions of them. The resulting device is structurally complex and expensive.